Google has issued a public apology to major advertisers after their spots were featured alongside content from hate preachers and other extremists.

"I would like to apologize to our partners and advertisers who might have been affected by their ads appearing on controversial content," Google executive Matt Brittin said at an industry conference in London on Monday. "We take our responsibilities to these industry issues very seriously."

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Several major companies and ad agencies have pulled their ads from Google and YouTube in recent days after a newspaper investigation showed their product pitches were being displayed on content including videos from former Ku Klux Klan leader David Duke.

The British government said last week that it would temporarily restrict its advertising on the company's platforms. It has been joined by The Guardian, Marks & Spencer and French marketing giant Havas, which represents clients including O2 and Royal Mail.

Google and Facebook own huge chunks of the digital advertising business, and brands are spending an increasing share of their ad budgets on their platforms.

Like other tech firms, Google relies on users to report inappropriate content posted to its platforms. Flagged content is then reviewed -- a process that Google says takes less than 24 hours in 98% of cases.

But the company admits it needs to do better. Brittin said that Google would set a higher standard for videos and sites against which advertisements appear.

He also said that brands would be given more control over where their ads appear.

"We are looking at accelerating changes to policy controls and enforcement that will help us to do better," he said. "We need to improve and we will do."

Martin Sorrell, CEO of the global advertising firm WPP, said that Google must do more.

"We have always said Google, Facebook and others are media companies and have the same responsibilities as any other media company," he said. "They cannot masquerade as technology companies, particularly when they place advertisements."

At least one market analyst has downgraded shares in Google from "buy" to "hold" over the issue.

"We think that the problems which have come to light will have global repercussions," said Brian Wieser of Pivotal Research Group. "Google will probably need to articulate goals that sound more like a zero tolerance policy."